The order in which you place your effects bears some thought; every element of your signal chain will effect what's come before it and be effected by what comes after it.
For example, imagine you've set up an amp stack in a cathedral and you're cranking out some fat, distorted power chords. Imagine the glorious, lush reverb that will come bouncing back at you in a room of that size. Then imagine taking that reverb and pushing it into the front of your overdriven amp. The amp will do what it's supposed to, it'll distort and compress the sound you've given it, and all of the complexity and space in that reverb will be lost.
This is where the effects loop comes in. It enables us to tap in to the signal chain after the preamp (where your tone is born), but before the power amp and speakers. This means you can put your amp anywhere you like in your chain of effects, rather than it always having to be the final stop.